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ГИДРАВЛИКА . ГЕОТЕХНИКА. ГИДРОТЕХНИЧЕСКОЕ СТРОИТЕЛЬСТВО
УДК 631.6(635) DOI: 10.22227/1997-0935.2021.8.1065-1076
Runoff irrigation practices and challenges in western lowlands of Eritrea Anghesom A. Ghebrehiwot1,2, Dmitry V. Kozlov2
1 Hamelmalo Agricultural College, National Higher Education and Research Institute; Keren, Eritrea; 2 Moscow State University of Civil Engineering (National Research University) (MGSU); Moscow,
Russian Federation
ABSTRACT
Introduction. The arid and semi-desert lowland agro-ecological zones of Eritrea experience lowprecipitation, much lower than the requirements forrobust agricultural production unless supplemented by properly functioning runoff irrigation systems. However, an in-depth understandingof the principles and practices of runoff irrigation, identification of itspotentials and challengesand come up with viable solutions is necessary.
Materials and methods. Qualitative and quantitative, descriptive and analytical research methodologies are applied. Primary and secondary data are used to identify existing constraints. Besides, global and regional databases are extensively utilized to fill information gaps.
Results. The total potential cultivable land of Eritrea amounts to 2.1 million ha, out of which 71.4 % is rainfed and 28.6 % is irrigation. But, the potential irrigable land as reported by FAO is much lower (187,500 ha), out of which 50,000 ha is within the Western Lowlands. Considering such disparities and the less likely scenario of lowest irrigation potential, the actual equipped for spate irrigation at national level would amount to only 33.6 %, meaning there are still a lot of possibilities for expansion. The causes of malfunctioning of the existing systems are associated to structural, operation and maintenance, and management. Lack of historical hydrological data is among the highly likely reasons, which in turn greatly affects hy-drological simulations. v m
Conclusions. Despitecommendable efforts made to expand the improved runoff irrigation in the Western Lowlands, most e е of the projectshavefailed to achieve the intended purposes. Thus, comprehensive and simple mathematical modelsfor ma- ü T king hydrological predictions have been suggested. 2. и
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KEYWORDS: runoff irrigation, spate hydrology, spate irrigation, water scarcity, water stress, Western Lowlands of Eritrea
FOR CITATION: Ghebrehiwot A.A., Kozlov D.V. Runoff irrigation practices and challenges in western lowlands of Eritrea. Vestnik MGSU [Monthly Journal on Construction and Architecture]. 2021; 16(8):1065-1076. DOI: 10.22227/19970935.2021.8.1065-1076 (rus.).
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Орошение аккумулированными ливневыми водами и проблемы управления водными ресурсами в западных 1 §
низменностях Эритреи
АА. Гебрехивот1,2, Д.В. Козлов2 С $
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1 Сельскохозяйственный колледж Хамельмало, Национальный институт высшего образования
и научных исследований; г. Кэрэн, Государство Эритрея;
2 Национальный исследовательский Московский государственный строительный университет 6
(НИУМГСУ); г. Москва, Россия ^ 0
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Введение. В пустынных и полупустынных низинных агроэкологических зонах Эритреи выпадает мало осадков, что • )
недостаточно для устойчивого сельскохозяйственного производства, если не использовать технологии орошения. ^ •
Поэтому необходимы глубокое понимание особенностей и технологии орошения аккумулированными ливневыми О О
водами, определение возможностей и существующих проблем такого орошения и, в конечном итоге, разработка с ^
методологии преодоления этих проблем. 3 6
Материалы и методы. Применены качественные и количественные, описательные и аналитические методы иссле- ^ Р
дования. Использованы результаты опроса целевой группы лиц, участвовавших в осуществлении гидротехнических . И
проектов по орошению аккумулированными ливневыми водами; вторичные данные, полученные в результате ана- ( §
лиза ведомственной документации; в условиях недостаточного информационного обеспечения общедоступные гло- л у
бальные и региональные базы данных. 3 К
Результаты. Общая площадь пахотных земель Эритреи составляет 2,1 млн га, из которых 71,4 % — богарные, 8 8
28,6 % — орошаемые. Площадь потенциально орошаемых земель (по данным ФАО) намного меньше (187 §00 га), 2 2
из которых §0 000 га находятся в Западной низменности. Такие статистические различия характерны и для факти- О о
чески орошаемых сельхозугодий. Даже если принять во внимание более низкие оценки ирригационного потенциала 1 1 (187 §00 га), фактическая площадь орошаемых земель будет составлять только 33,6 % от общего потенциала оро-
© Anghesom A. Ghebrehiwot, Dmitry V. Kozlov, 2021
Распространяется на основании Creative Commons Attribution Non-Commercial (CC BY-NC)
шения. Это указывает на возможности для развития орошаемого земледелия в регионе. Анализ итогов проведенного опроса специалистов водного хозяйства показал наличие ошибочных управленческих решений, эксплуатационных и технических проблем в работе гидротехнических сооружений, связанных в том числе с отсутствием многолетних гидрологических наблюдений, что в значительной мере влияет на результаты гидрологического моделирования. Выводы. Несмотря на значительные усилия по разработке и внедрению ирригационных проектов в Западной низменности, большинство из них не достигли намеченных целей. Предложено использовать массивы данных глобального реанализа климата, комплексные и простые физико-математические модели формирования стока.
КЛЮЧЕВЫЕ СЛОВА: орошение аккумулированными ливневыми водами, поливное орошение, водосбор, дефицит воды, водный кризис, Западные низменности Эритреи
ДЛЯ ЦИТИРОВАНИЯ: Гебрехивот A.A., Козлов Д.В. Runoff irrigation practices and challenges in western lowlands of Eritrea // Вестник МГСУ. 2021. Т. 16. Вып. 8. С. 1065-1076. DOI: 10.22227/1997-0935.2021.8.1065-1076
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INTRODUCTION
Climate change is likely to affect the availability, quality and quantity of water. The hydrological changes, induced by climate change, produce more challenges to the sustainable management of water resources. Much of the impacts of climate change have already been manifested in the tropical zones (e.g., sub-Saharan Africa) [1]. In sub-Saharan Africa, water resources are subjected to high hydro-climatic variability over space and time, and are a key constraint on the region's continued economic development. Countries in this region are environmentally and socio-economically vulnerable to disasters and climate change, and many of them are expected to experience increasing water shortages and desertification in the future1, 2. Conspicuously, food security, health, economy, energy, and ecosystems are all water-dependent and thus vulnerable to the impacts of climate change. The impacts of climate change on sub-Saharan Africa's water resources thereupon food security and human health are already acute [2, 3]. Therefore, climate change adaptation and mitigation through water management is critical to sustainable development, and essential to achieving the 2030 Agenda for Sustainable Development, the Paris Agreement on Climate Change and the Sendai Framework for Disaster Risk Reduction.
Agriculture is the leading consumer of water and mainstay of economic growth in many sub-Saharan African countries. More than 60 % of the labor forces are involved in agriculture related activities. But, agricultural production and the economy of Africa largely depend on sparse and unreliable seasonal rainfall. Most countries in the region face a combination of high hydrological variability, a lack of investment in water infrastructure and weak water governance3. There are
1 The United Nations World Water Development Report 2020: Water and Climate Change. Paris, UNESCO, 2020.
2 IPCC. Africa, in Climate Change 2014: Impacts, Adaptation and Vulnerability. Part B: Regional Aspects: Working Group II Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge, UK, Cambridge University Press, 2014. Pp. 1199-1266.
3 UN. Sustainable Development Goal 6: Synthesis Report 2018
on Water and Sanitation. New York, USA, 2018.
several terms that are used to indicate the level of water availability, such as water scarcity, water stress, water depletion, inter-annual and seasonal variability, groundwater level decline, drought, etc. Among them, water stress and water scarcity are given more emphasis in this discussion.
Water scarcity is a gap between available supply and demand of freshwater in a specified domain, under prevailing institutional arrangements and infrastructural conditions. Causes of water scarcity are many and inexplicably interlinked4. The factors that affect the annual available supply of water can be natural or anthropogenic: the natural factors include annual volumes of flow, climatic variability, geological and geomorphological conditions, uneven distribution of water, etc.; anthropogenic factors are water control or storage structures. But, factors affecting water demand are all anthropogenic, such as economic, population growth, urbanization, etc. According to FAO5, water scarcity has three dimensions: physical — scarcity in availability of fresh water of acceptable quality with respect to aggregated demand (i.e., physical water shortage); infrastructural (economic) — scarcity due to the lack of adequate infrastructure, irrespective of the level of water resources, due to financial constraints; and institutional — scarcity in access to water services because of the failure of institutions in place to ensure reliable supply of water to users. Water scarcity is signaled by unsatisfied demand, tensions between users, competition for water, over-extraction of groundwater and insufficient flows to the natural environment4. The best-known indicator of national water scarcity is done based on the amount of annual total renewable water resources (TRWR) per capita. As such, countries or regions are considered to be facing absolute water scarcity if their TRWR is less than 500 m3 per capita, chronic water shortage if TRWR are between 500 and 1,000 m3 per capita, regular water stress if TRWR between 1,000 and 1,700 m3 per capita, and occasional
4 FAO. Coping with water scarcity: An action framework for agriculture and food security. FAO Water Reports. Rome, Italy, 2012.
5 FAO. The state of the world's land and water resources for food and agriculture (SOLAW) — Managing systems at risk. Rome and Earthscn, London, 2011.
or local water stress if TRWR is more than 1,700 m3 per capita [4].On the other hand, water stress measures total annual water withdrawals (municipal, industrial, and agricultural) expressed as a percentage of the total annual available blue water6. Higher values indicate more competition among users; hence, less than 10 % or a score of (0-1) is low stress, 10-20 % (1-2) is low to medium stress, 20-40 % (2-3) is medium to high stress, 40-80 % (3-4) is high stress, and greater than 80 % (4-5) extremely high stress.
About 66 % of the African land mass is arid or semi-arid, and more than 300 million of the 800 million people in sub-Saharan Africa (in 2015) lived in absolute or chronic water stress environments; that is, they had less than 1,000 m3 per capita. The reasonsmainly stemmed from economic and/or institutional scarcity and to a lesser degree from physical scarcity [5]. While water stress is only 3 % (low stress) in sub-Saharan Africa as compared to world (13 % — low to medium stress), the region has a high prevalence of severe food insecurity (29 %). This is due partly to unevenly distributed water resources that are poorly managed, lack of investment and are affected by conflict and natural hazards, such as droughts and floods3. These situations are further complicated by the drivers of the water crisis (for example, demographic growth, economic development, urbanization, pollution, etc.). The Horn of Africa (Eritrea included), where farming constitutes more than the continental average (over 80 %) of national economies, has been experiencing effects of recurrent climate change evidenced by declining levels of water bodies, unpredictable rainfall patterns, prolonged drought, erratic floods, landslides and changing temperatures [6]. Thus, economies and livelihoods of these nations are facing unprecedented vulnerability to climate change. Lack of sufficient water means there will be community-based conflicts over food and water. In fact, future projections on food and water indicate that this situation will continue to get worse due to increase in demand7.
People have always tried to control and store seasonal and irregular water flows in order to have access to water, limit the damages of floods and overcome droughts. Moldenand others8 proposed three steps to cope up with water scarcity: development, utilization, and reallocation. Firstly, managing the supply can be done by increasing access to conventional and non-conventional water resources, including dam storage,
6 Luo T., Young R., Reig P. Aqueduct projected water stress rankings. Technical note. 2015.
7 IPCC. Climate Change and Land: IPCC Special Report on Climate Change, Desertification, Land Degradation, Sustainable Land Management, Food Security, and Greenhouse gas fluxes in Terrestrial Ecosystems. Summary for Policymakers. 2019.
8 Molden D., Sakthivadivel R., Keller J. Hydronomic Zones
for Developing Basin Water Conservation Strategies. Research
Report No. 56, Colombo, Sri Lanka, 2001.
groundwater withdrawals, harvesting rainwater, reusing wastewater and drainage water, desalination of brackish or salt water, and the use of fossil groundwater. Secondly, water quality deteriorates and aquatic ecosystems are damaged due to reduced water quality and quantity. Thus, water policies focus on improving water management and conservation are necessary. Thirdly, water has become a rare commodity and is no longer sufficient to satisfy the aggregated demand from all sectors. Policies are directed towards the economic optimization of water, with emphasis on reallocation of water from low value to high value uses. The three major water policy domains, where supply enhancement and demand management can be applied, are water, agriculture and national food security4.
As noted earlier, Eritrea is located in one of the driest parts of Africa with scarce fresh water resources. The population of Eritrea has beendifferently reported between 3.41 to more than 5million. With the assumption of 3.41 million in 2019, the population of Eritrea is projected to be 4.24 million by 2030, 6 million by 2050 and by the end of the century it will be 9.06 million9. In 2017, out of the total population, 90 % live in rural areas and the remaining 10 % in urban areas. The long-term average annual precipitation and renewable water resources are 384 mm (45.16 km3 per year) and 7.31 km3 per year, respectively; the annual internal and external renewable water resources represent 2.8 km3 and 4.52 km3, respectively. The annual TRWRis also differently reported between 1,399 m3 per capita and 2143 m3 per capita10. In both scenarios, TRWR is by far lower than continental and global annual averages whose corresponding values are 3,133 and 5,675 m3 per capita, respectively. According to World Resource Institute (WRI)6, Eritrea's water stress score is 3.34 and 3 for 2010 and 2040, respectively. This means that even though some improvement is expected to occur in the future, the water stress level willremain within the range of40-80 % (that is, high stress) due to increase in demand and other factors. Therefore, the water stress and water scarcity indices demonstrate that Eritrea is literally one of the countries with high or extremely high water vulnerability. Furthermore, Eritrea's geographical location thereupon its climate on one hand, and decades-long warson the other hand, renders it vulnerable to persistent water shortages [7] and food deficits. A recent study on the degree of vulnerability of Eritrea to climate change demonstrated that more than 88 % of the farming households were found to be vulnerable or highly vulnerable to climate change as a result of the combined effect of their exposure to external factors, sensitivity to internal factors, and lower adaptive capacity [8].
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9 UN. World Population Prospects 2019: Volume II: Demographic Profiles. 2019.
10 FAO-AQUASTAT. Global Information System on Water and Agriculture: Water resources. 2020.
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Over 72 % of the population in Eritrea is engaged in subsistence agriculture. With future increase in population, annual food requirement per person need to be increased. According to FAOIO, the total potential arable land in Eritrea is estimated at 1.5 million ha, with nearly 50 % of which is found in the lowlands of arid and semi-desert agro-ecological zones. These two zones receive on average, 200 to 400 mm of rainfall, which is low for agricultural production unless otherwise supplemented by irrigation [9, 10]. Thus, an increased agricultural production can be achieved through improving the existing irrigation systems (for example, pressurized irrigation and runoff irrigation- also known as "spate irrigation") in the potential irrigable areas. Therefore, the present study aimed at achieving the following objectives: 1) to understand the concepts and practices of runoff irrigation; 2) to explore the development potentials and existing challenges of runoff irrigation in Western Lowland plains of Eritrea; 3) to suggest methodologies that can help to overcome the challenges.
MATERIALS AND METHODS
As has been noted earlier, the study area focused primarily on the runoff irrigation systems established in the Western Lowlands, which can be better described by the topography and climate of Eritrea. The landscape of Eritrea can be broadly classified as the Eastern Lowland, Central Highland, and Western Lowland. The Eastern Lowland with an elevation of 0 to 1500 m above mean sea level (msl) and being part of the Great Rift Valley stretches along the Red Sea for about 800 km with a width of about 30 to 100 km. The Western Lowland with an elevation of 500 to 1,500 m above msl comprises the area that stretches from south to northwards along the Sudan border. The Eastern Lowland and Western Lowland are hot and dry regions, with sparse settlements. The Central Highland represents areas with high altitudes and temperate humid climate with 1500 to 2500 m above msl. It is the most densely populated region in the country. The highest point is EmbaSoira with an elevation of 3018 m above mean sea level (msl) and the lowest point is situated in the Danakil depression at Lake Kulul near the Djibouti border with an elevation of 75 m below msl [9]. Fig. 1a shows the long-term annual rainfall distribution of Eritrea. Most of the Western Lowland and almost the whole eastern lowland areas receive an annual rainfall of less than 400 mm. The area experiences hot to very hot climate with a mean annual temperature of more than 27 °C. Rainfall is received from April/May to September/ October provided by the south-western monsoons.
Eritrea has five major river basin systems, namely Red Sea, Barka-Anseba, Mereb-Gash, Danakil depression, and Setit (Fig. 1, b). From south to north, the main westward drainages of the Ethiopian-Eritrean Highlands are the Setit, Mereb-Gash, Barka-Anseba rivers. Each drains west-northwest to swing abruptly northwards as
they cross the line of the Barka lineament. Barka-Anseba river basin is a basin formed as a result of a confluence of two rivers, namely Barka and Anseba, close to the Sudan border in the extreme northwest of Eritrea. Due to the geography of Eritrea and thereupon its climate, the flows through most of the rivers are highly seasonal (ephemeral) with the exception of Setit — a Perennial River. The Western Lowlands plains are predominantly situated within the Barka-Anseba and Mereb-Gash river basins and Set it basin to some extent.
To achieve the objectives, qualitative and quantitative, descriptive and analytical research methodologies were used. Interviews and questionnaire survey were undertaken with targeted people, who had been involved in the implementation of various runoff irrigation as well as other water resources projects in Eritrea, with the intention of identifying existing challenges and their solutions. Secondary data obtained from various ministries were also utilized, notably the baseline survey from the Ministry of Agriculture (MoA). Besides, global and regional databases, which are freely available for public domain such as FAO-AQUASTAT, World Resources Institute (WRI), etc., were extensively employed to fill the gap in data insufficiency. Eventually, the collected information was analyzed and discussed as presented in the ensuing section.
Concepts of spate irrigation
Runoff irrigation or spate irrigation is a type of surface irrigation system usually practiced in arid and semi-arid regions where evapotranspiration greatly exceeds rainfall. It is defined as "an ancient irrigation practice that involves the diversion of flashy spate floods running off from mountainous catchments where flood flows, usually flowing for only a few hours with appreciable discharges and with recession flows lasting for only one to a few days, are channeled through short steep canals to bunded basins, which are flooded to a certain depth" [11]. The structures used to divert the runoff flood to irrigation fields are made up of earthen, brushwood, gabion or concrete structures. The irrigation fields are flooded at least two to three times to a depth not less than 50 cm during rainy season. This process supplements the soil moisture content in the deep alluvial soils formed from the sediments deposited during previous irrigations. As such, crops can be grown from one or more irrigations with reduced risk of low yield during dry periods [6, 11].
The fraction of rainfall that flows over the landscape from higher to lower elevations is known as runoff. Runoff is usually associated with negative implications, such as erosion, water loss, etc. It can, however, be used for surface irrigation during rainfall events. It is accomplished by diverting runoff water into plots where crops are grown; each plot is surrounded by earthen bunds. Thus, the rainfall irrigation system provides the advantage of developing agricultural activities in areas where such activities are impossible without it. The amount of water that can be collected during a rainfall
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Fig. 1. Spatial distribution of annual rainfall (a) and major river basins of Eritrea (b)
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event depends on rainfall and watershed characteristics. The main difference between runoff irrigation and conventional irrigation is that the timing and the amount of the application cannot be determined a priori. The variability in available water can be minimized by adjusting the size of the plots receiving runoff. Thus, in order to model the productivity of a crop in such a system, it is necessary to model the diverse processes involved such as rainfall, infiltration, surface flow and consumptive water use [9, 11, 12].
Historically, runoff irrigation is believed to be the oldest form of irrigation. It has been practiced for millennia in regions with arid and semi-arid climate setting. The most well-known runoff irrigation systems are found in the Arabian Peninsula, notably in Yemen, where it dates back to 2000 years, the Negev Desert region, which was built during the Israeli, Nabataean and Roman-Byzantian periods going back to 1300 to 2900 years. Nowadays, it is foundin the Middle East, North Africa, West Asia, East Africa and parts of Latin America [13]. In Eritrea, the spate irrigation practice is alleged to have been introduced in the beginning of the 20th by Yemeni migrants. It has been widely practiced form of agriculture in Eastern Lowlands and some parts of Western Lowlands of Eritrea [9]. The FAO database10 also indicates that runoff irrigation has been extensively practiced in countries with arid and semi-climates (Table 1). Currently, Pakistan is the leading country in spate irrigation practices (720,000 ha), followed by Yemen (218,000 ha) and Ethiopia (200,000 ha). However, the highest percentage of area under spate irrigation to total irrigation is found in Somalia (75 %) and Eritrea (61 %). Here, it is worth reporting the fact that the date of data collection is variable as variable as two to threedecades.
Runoff irrigation system consists of two major components: catchment and field areas. While the catchment area generates runoff, the field grows crops
as a result of the application of diverted runoff. Two basic requirements must be met to establish a runoff irrigation system: (1) presence of mountainous or hilly topography with adjacent low-lying fields on the same plain and (2) a field with deep soils capable of storing sufficient moisture. The former generates runoff water that can be directed to the field so as to supply the crops during periods of no precipitation. Two different types of runoff irrigation systems are employed, depending on the slope of the terrain: micro and macro. Micro-catchment system is for locations where the catchment area and the fields lie neighboring to each other on the same plain. Macro-catchment system is a catchment area located on a slope with the usually terraced fields at the foot of the slope. The ratio of the catchment area to field area in micro-catchment runoff system varies from 1:1 to 10:1 and in macro-catchment systems from 10:1 to 100:1. For further details on the advantages and disadvantages of each method can be referred to the works of [9, 12].
Water is used for food production in various ways, including for agriculture, livestock and inland fishery production. Water use in agriculture ranges from essentially rainfed, relying on soil moisture from rainfall, to entirely irrigate. Rainfed agriculture covers 80 % of the world's cropland and accounts for the major part (60 %) of food production. Table 2 shows land cultivated and equipped for irrigation and agricultural water withdrawals in 2017. Irrigated agriculture covers about 22 % of cultivated lands. Water withdrawals from surface and groundwater resources for irrigation currently amount to 2,875 km3 per year, which represents 72 % of all water withdrawals in the world. In many drier countries, irrigation water use usually accounts for more than 90 % of total water withdrawals1. For instance, the agricultural water withdrawal in Eritrea is 95 %. Worldwide, the total cultivated area is about 1565 million ha, which is approximately 12 % of the total land area.
Table 1. Area equipped for spate irrigation in selected countries1'
Country Area under spate irrigation (in 103 ha) Area under total irrigation (in 103 ha) % of total irrigation
Algeria 53 1230 4
Eritrea* 63 103.9 61
Ethiopia 200 858.3 23
Mongolia 27 84.3 32
Morocco 62.2 1,520 4
Pakistan 720 19,990 4
Somalia 150 200 75
Sudan 126 1852 7
Tunisia 27 486.6 6
Yemen 218 680.1 32
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Table 2. Land cultivated and equipped for irrigation agricultural water withdrawals (in 2017)10
Region Total cultivated land Land equipped for irrigation Land equipped for irrigation as % of total cultivated land Total water withdrawal Agricultural water withdrawal Agricultural water withdrawal as % of total water withdrawal
106 ha 106 ha % km3/year km3/year %
Africa 276 16 6 236 186 79
Americas 378 55 15 812 417 51
Asia 589 238 40 2,641 2,168 82
Europe 288 26 9 299 90 30
Oceania 34 3 9 22 14 63
World 1,565 338 22 4,010 2875 72
Eritrea 0.692 0.021 3 0.58 0.55 95
Presently, irrigation only covers 338 million ha, that is, 24 % of the world's arable land, but is responsible for around 40 % of world crop output. Irrigation uses about 70 % of waters withdrawn from global river systems. In different regions of the world depending on the local climatic and other factors different types of water management with different levels of service will be appropriated. The extent of the role played by water management in different continents and types of countries as far as agricultural production is concerned is presented in Table 3. Eritrea covers an area of 117,600 km2 and has a coastline of over 1,000 km. The total cultivable area is estimated at around 1.5 million ha. In 2017, the total cultivated area was 692,000 ha, out of which 690,000 ha arable land and 2,000 ha permanent crops (Table 3). Most of the country consists of savannah, steppes and desert, particularly in the south-western lowlands and in the east near the Red Sea. The highlands, where altitudes range between 1,500 and 2,000 m above mean sea level (msl) are among the oldest areas cultivated by humans and are showing signs of overuse. The population density with reference to total area and arable land are estimated to be 29 and 493 persons per km2, respectively. This is
actually lower than the values corresponding to continental, global and even least developed values.
Of the country's land area of 11.76 million hectares only about 417,000 (3.4 %) are under rainfed cultivation or fallow, while mere 22,000 ha (0.18 %) are irrigated. Nevertheless, agriculture is the most important sector of the Eritrean economy, accounting for about 50 % of the gross domestic product and the bulk of export earnings. In Eritrea the greatest part of the agricultural sector is run by subsistence farmers, who cultivate more or less from one to three hectares depending on the availability of land. Main source of their production are a wide range of rainfed crops grown in the above mentioned different agroecological zones of the country. Among the most important of these are cereals, pulses, oil seeds, fibre crops, spices, medicinal plants, and others. However, some farmers, where water is available, grow also vegetables under irrigation during the winter dry season.
A properly managed and developed irrigation system is one of the methods used to increase food production and thereupon poverty reduction in arid and semi-arid regions; it can enhance food security, promote economic growth and sustainable development, create
Table 3. Population density with reference to total area and arable land 9 10, 11
Region Total area (106 ha) Total population (millions in 2017) Arable land (106 ha) Population density in persons/km2 with reference to
total area arable land
Africa 3,033 1,300 241 43 539
Americas 4,065 1,014 351 25 289
Asia 3,200 4,600 498 144 924
Europe 2,333 747 273 32 274
Oceania 856 42 32 5 131
World 13,487 7,703 1395 57 552
Eritrea 11.76 3.412 0.69 29 493
Least developed 2,078 1,000 183 48 546
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employment opportunities, improve living conditions of farmers as well as recharge groundwater. Runoff irrigation can be classified into traditional (uncontrolled) and improved (controlled). The former relies heavily on sand, stone and brushwood spurs and earthen guide bunds, which are damaged frequently; flexible, based on locally available material, relatively efficient, and limit diversion of high flows and high sediment loads. The downside of such system is that it requires enormous input of labour and resources for maintenance and reconstruction. In the improved (controlled) type, more robust and more permanent diversion and water control structures are usually constructed, reducing excessive maintenance burden. This technique too has disadvantages such as low economic returns and diversion and sedimentation problems.
According to FAO [11], there are four common features of spate irrigation schemes: ingenious diversion systems, built to capture short floods but also designed to keep out the larger and most destructive water flows; sediment management — the flood water has high sediment loads that would otherwise fill reservoirs and clog intake structures and distribution canals; soil moisture conservation; and complex social organization. Understanding the hydrology (also called spate hydrology in this context) of the catchment area is critical to develop spate irrigation system. The potential yield of spate irrigation systems, the design of diversion structures and canals and the area to be potentially irrigated is governed by the hydrology of the area [14]. Spate hydrology is characterized by a great variation in the size and frequency of floods, which directly influence the availability of water. Spate floods can have very high peak discharges that make it very difficult to determine the volumes of water that will be diverted to irrigation fields. Various studies conducted in Saudi-Arabia, Yemen and Eritrea have shown that very poor correlation between observed rainfall and runoff [9, 11, 15, 16]. Estimates of flood discharges and runoff volumes derived from conventional rainfall/runoff models are therefore of limited use in spate systems. The important hydrological parameters that need to be determined include estimating mean annual runoff using a runoff coefficient, runoff volume, the
proportion of annual runoff that is to be diverted, and design flood (discharge). The mean annual runoff usually accounts for 5 to 10 % of the mean annual precipitation. Another important characteristic of spate floods is that they transport very high sediment loads (2 to 3 times more than runoff in perennial rivers. Therefore, management of sedimentation is a key factor in spate irrigation design.
RESULTS OF THE RESEARCH
The term irrigation potential in this study is used to describe total irrigation practices, such as pressurized, border, spate, etc. or a particular type of irrigation, for instance, spate irrigation. Eritrea's total irrigation potential can be better understood by evaluating the water availability or runoff volume in the major river basins as presented in Table 4. Accordingly, the Setitbasin has the highest irrigation potential (225,000 ha) followed by Red Sea and Mereb-Gash with lower runff coefficient. Nevertheless, when annual runoff volumes are considered, Mereb-Gash is the leading one (532 million m3) as a result of highest runoff coefficient. The Barka-Ansebaand Denakil depression lack credible information about their irrigation potentials. Although the annual rainfall volume of the Barka-Anseba basin is estimated at 14,815 million m3, the annual flow volume is estimated at only 41 million m3. This is perhaps because of greater values of infiltration rates into the sandy plains of the river valleys. Generally, the Setit, Barka-Anseba, and Mereb-Gash river basins, whose downstream plains located in the Western Lowlands, are among the major river basins with the highest irrigation potential. The total potential area where runoff irrigation system can be developed at national level is estimated to be 187,500 ha, out of which nearly 50,000 ha are found along the major river basins in the Western Lowlands and the rest along the coastal plains in the Eastern Lowlands [12]. In fact this estimate seems to underestimate far below the actual potential reported by the MoA, Eritrea.
Table 5 represents the agricultural potentials of Eritrea and its current status. Out of the total land area of Eritrea, total potential cultivable land amounts to 2.1
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Table 4. Runoff flow volumes and irrigation potentials of major river basins [12]
Area, km2 Mean annual Mean annual rainfall Annual flow Mean annual Potential irrigation land (in 103 ha)
Basin rainfall, mm volume, 106 m3 volume, 106 m3 runoff coefficient
1 2 3 = 1x2 4 5 i i 3
Red Sea 44,689 350 15,641 444 0.028 137
Barka-Anseba 39,506 375 14,815 41 0.003 N/A
Mereb-Gash 23,455 600 14,073 532 0.038 67.56
Danakil 10,532 200 2,106 135 0.064 N/A
Setit 7,517 650 4,886 49 0.010 224.6
Total 125,699 — 51,521 1,201 0.023 429.16
Note: N/A — not available.
million ha; 71.4 % (1.5 million ha) of it is rainfed and 28.6 % (0.6 million ha) is irrigable. Currently, only 416,000 ha (28 %) of the potential cultivablerainfed is actually cultivated whereas out of the potential cultivable irrigated land, 104,000 ha (17 %) is actually irrigated. As noted earlier, FAO-AQUASTAT database estimations are much lower than the official reports from MoA. Such disparities are also reported in actual area equipped for irrigation. For example, actual area equipped for spate irrigation in Eritrea is estimated to be between 17,490 and 63,000 ha by FAO-AUASTAT and MoA, respectively. In the latter case (in 2013), the total actual area equipped for spate irrigation was only 33.6 % of the total irrigation potential estimated by FAO-AQUASTAT (187,500 ha) or 9.3 % of the irrigation potential estimated by MoA (0.6 million ha). This shows that there are still possibilities for expansion of the existing systems. The reason for disparity in irrigation potential estimation could be related to the timing of data collection. However, due to the incompleteness of the information from either of them, both data sources are employed for analysis. Table 6 shows irrigation potential by sources of water; hence,the actual area equipped for irrigation is predominantly dependent on surface water than groundwater. The total area equipped for irrigation by groundwater and surface water is 21,590 ha, of which 82 % uses surface water and the remaining 18 % depends on groundwater10.
Table 5. Agricultural potential and current status in Eritrea
Agricultural land Area (in 103 ha)
Country area 11,760
Potential cultivable land
Rain-fed 1,500
Irrigation 600
Total potential cultivable land 2,100
a. Actual cultivated rain-fed 416.1
b. Actual area equipped for irrigation
Spate irrigation 63
Dams 8.6
Pressurized irrigation 5.4
Well irrigation (open channel) 26.9
Total actual area equipped for irrigation 103.9
Total actual cultivated (a and b) 520
Note: Source: MoA, 2013.
Table 6. Irrigation potential by groundwater and surface water11
The performance of a spate irrigation system, irrespective of its scale, can be generally described in terms of equity, regularity and reliability of water delivery, and durability [9, 17]. Fadul and others [6] revealed that the most effective measures include pre-flood preparedness, risk sharing measures through water and land management during and after flood by water user associations, crop management by farmers, and flexibility in operation by water managers. On the other hand, like any engineering interventions, complete and partial failures have had serious implications on the economy, livelihoods of communities and the environment. Various literatures [18, 19] found that users of state-administered irrigation systems are less aware of the implementation of rules and sanctions for noncompliance, due to externally enforced rules. In this regard, despite the promising efforts made to establish improved spate irrigation systems in potential Western Lowlands, their overall performance is low. For example, even though the spate irrigation systems doubled quantitatively (from 16 to 35) over the period of 2003 and 2016 in Gash-Barka region alone, none of them were fully functional as of 2016. Fig. 2 shows the complete and partial failure of the runoff diversion structures in Hashenkit and Golij (Engulit). After few initial years of operation, the Hashenkit project failed miserably (Fig. 2, a), following structural failure of its components. Thenceforth, inundation of the nearby plains including residential areas has been reported during extreme floods. Similarly, Golij diversion structure was severely out of order (Fig. 2, b): upstream side filled with sediments; gates damaged and/or blocked, and downstream tail beginning to scour. Failures of these types are prevalent in almost all the projects. The question that requires to be answered is, therefore, what could be the motive for such failures.
To somehow answer the aforesaid question, we undertook a survey on targeted personson the causes, reasons and consequences of runoff diversion systems in the Western Lowland and summary of the survey is presented in Table 7. As such, three causes are identified: structural design and construction, operation and maintenance, and management. The first problem mainly stems from incorrect hydrological predictions from ungauged catchments, leading ultimately to over-design and under-design of hydraulic structures and accumulation of sediments. These predictions include peak floods, rainfall volume, sediment load, etc. Practical experiences
Region Area equipped for irrigation, ha % of area equipped for irrigation Full/partial controlled irrigation, ha % of full/partial controlled irrigation Spate irrigation, ha Irrigation potential, ha
Groundwater 3,960 18.3 3,960 96.6 — —
Surface water 17,630 81.7 140 3.4 17,490 —
Total 21,590 100 4,100 100 17,490 187,500
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from one of the earliest post-independence modern technology-based spate irrigation systems in Shieb (Eastern Lowland of Eritrea)also showed the following problems [9]: uncertainty in estimating extreme flood return period; a limited knowledge of appropriate concept for developing improved spate irrigation system and the technical problems dealing with destructive floods and high sediment; the lack of ownership and adequate provision for operation and maintenance after completion of improved system; and adoption of new technologies
and concepts by the farming community needs more time, personnel and budget than expected. More or less the same problems have persisted in the spate irrigation systems established thenceforth in the Western Lowland plains (for example, Hashenkit, Golij-Engulit, Mogoraib, etc.). These problems are seemingly common in arid and semi-arid regions where spate irrigation is widely practiced. In most cases, the main source of risk is the extreme variability of rainfall, causing unpredictable flows regarding volume, timing andduration [20].
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Table 7. Causes, reasons and consequences of runoff irrigation systems in the Western Lowland
Cause Reason Consequence
Structural design and construction Designs of the main diversion structures are not based on long-term measured data of flood discharge, rainfall volume of catchments, stream flow discharge, sediment load, etc. Underestimation and overestimation leads to economically and structurallyunviablesystems; accumulation of sediments at the upstream of the headworks and main canal intakesthat substantially reduce the amount of water being diverted to the fields
Operation and maintenance Inadequate funds for repair and maintenance of gates, weirs, canals, etc.; lack of awareness, communication, coordination and leniency Affects the smooth functioning of the system ultimately leading to total abundance of a project: deposition of excess sediments can block and damage gates, etc.; degradation of irrigation infrastructure; and inequity in access to water.
Management Lack of proper planning and technical know-how, absence of follow up by experts and extension services, lack of regulatory mechanisms, and weak organization of beneficiaries Inefficient with low economic return system; and long-term socio-economic and environmental impacts
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CONCLUSION AND DISCUSSION
Runoff irrigation or spate irrigation is a type of surface irrigation system usually practiced in arid and semi-arid regions where evapotranspiration greatly exceeds rainfall. This system is believed to have been introduced more than century ago to the coastal Eastern Lowlands of the country. The Western Lowlands of
Eritrea with arid and semi-arid climate also has enormous spate irrigation development potential. As such, commendable endeavors have been attempted to establish projects of that kind since recently so as to enhance food security, economy and livelihood of communities in the immediate proximities. However, this study indicated that these projects, which are almost developed in hydrologically ungauged catchments, have hardly
achieved their intended purposes so far. This is possibly due to factors that are inextricably linked: structural design and construction as a result of absence of and/or insufficiency of hydrological data, operation and maintenance, and management. Obviously, viable solutions to these issues require multi-dimensional and multi-disciplinary approaches. The results from the survey on targeted persons was in-line with the hypothetical assumption of lack of hydrological data could be the main cause of runoff irrigation systems in the Western Lowlands. Thus, assuming the hydrological predictions as a highly likely plausible scenario, the following approaches are suggested:
• to use global reanalysis datasets as forcing data in physically-based mathematical models for continuous event simulations of flows (e.g. SWAT and MIKE). Results from these models will help us to determine annual runoff volume which is critical for determining the irrigation command areas and its management;
• to estimate single event-based design flood hydrographs using physiographic and geomorphologic catchment characteristics that can be obtained from topographic maps or remote sensing-based digital elevation models. This will be used for designing hydraulic components such as canals, spill-ways, water intakes, and gates.
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B i o n o t e s : Anghesom A. Ghebrehiwot — Lecturer of the Hamelmalo Agricultural College; National Higher Education and Research Institute; Keren-397, Keren, Eritrea; postgraduate student of the Department of Hydraulics and Hydrotechnical Engineering; Moscow State University of Civil Engineering (National Research University) (MGSU); 26 Yaroslavskoe shosse, Moscow, 129337, Russian Federation; bahghi2012@gmail.com;
Dmitriy V. Kozlov—Doctor of Technical Sciences, Professor, Head of the Department ofHydraulics and Hydraulic Engineering; Moscow State University of Civil Engineering (National Research University) (MGSU); 26 Yaroslavskoe shosse, Moscow, 129337, Russian Federation; SPIN-code: 5878-6674; Scopus: 36787104800, ResearcherlD: B-4808-2016, ORCID: 0000-0002-9440-0341; KozlovDV@mgsu.ru.
Поступила в редакцию 12 июня 2021 г. Принята в доработанном виде 25 августа 2021 г. Одобрена для публикации 25 августа 2021 г.
Об авторах : Ангхесом Ллемнгус Гебрехивот — преподаватель сельскохозяйственного колледжа Ха-мельмало; Национальный институт высшего образования и научных исследований; Кэрэн-397, Кэрэн, ® ® Государство Эритрея; аспирант кафедры гидравлики и гидротехнического строительства; Национальный ис-
О з следовательский Московский государственный строительный университет (НИУ МГСУ); 129337, г Москва,
Е J2 Ярославское шоссе, д. 26; bahghi2012@gmail.com; j
U (О Дмитрий Вячеславович Козлов — доктор технических наук, профессор, заведующий кафедрой гидрав-
<0 ф лики и гидротехнического строительства; Национальный исследовательский Московский государственный
2 IE строительный университет (НИУ МГСУ); 129337, г. Москва, Ярославское шоссе, д. 26; SPIN-код: 5878-6674,
0 -¡5 Scopus: 36787104800, ResearcherlD: B-4808-2016, ORCID: 0000-0002-9440-0341; KozlovDV@mgsu.ru.
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